With seismic survey apparatus as proposed hitherto the transmitting and receiving transducers are either fixed to opposite sides of a marine vessel or are incorporated in small floating units or in a common housing or separate housings located below the sea surface and towed by a marine vessel. A visual recording is made by a pen or stylus arranged to respond to output signals produced by the two transducers. The pen or stylus may be operated from the electrical or mechanical outputs of the transducers directly, or the electrical or mechanical outputs of the transducers may be recorded and the pen or stylus operated by "playing back" the recording.
Since such seismic survey apparatus is dependent on the time which elapses between the transmission of the original sound wave and the receipt of the reflected sound waves it is important that the two transducers are vertically stable and therefore, they can only be operated successfully in extremely calm water, i.e. a wave motion or swell of less than half a meter. If the wave motion or swell is more than one meter the value of the recording may be seriously reduced and when the wave motion or swell is two or three meters the recording may be of little value. Therefore, it will be appreciated that with even normal weather conditions in and around coastal waters, it is not possible to use such known seismic survey apparatus for a considerable part of the time.
An object of this invention is to provide seismic survey apparatus which reduces the above-mentioned difficulty.